Antennas radiate and receive intentional and unintentional electromagnetic signals. The unintentional signals, also known as field coupling or electromagnetic interference, may result from current-carrying traces, wires and other conductors, as well as from other antennas in a same or a different antenna module or structure. Unintentional signals associated with current-carrying traces, wires and other conductors can be minimized through proper circuit design and board layout, including the use of multilayer printed circuit boards with separate ground planes and/or the use of electromagnetic interference (EMI) shielding.
As illustrated in FIG. 1, conventional EMI shielding 100 uses a highly conductive metal plate, sheet or layer 110, inserted between one or more antennas and one or more circuits, to attenuate incident electric fields 112 by reflecting electric fields 114 and absorbing a portion of the electric fields. A portion of the electric fields is transmitted 116_1. There may also be internal reflections of the electric fields 118 that give rise to additional transmitted electric fields 116_2. An amount of attenuation of the electric fields will depend on factors such as a frequency and wavelength of the electromagnetic signals, the conductivity and permeability of the metal, its distance from the antenna and, if the wavelength of the electromagnetic signals is on the order of the thickness of the metal plate, sheet or layer 110, a thickness of the metal plate, sheet or layer 110. EMI shielding may be a simple metal sheet or foil layer, or an enclosure, such as a Faraday cage.
Unintentional electromagnetic signals associated with other antennas are common since multiple antennas are often implemented in close proximity. A high isolation of a respective antenna is often necessary to achieve good performance (a high signal-to-noise ratio, a low bit error rate, etc.) in a communication system. There are a variety of conventional techniques for improving the isolation between two or more antennas. One such approach isolates a transmit and a receive path in the communications system, for example, by using a transmit-receive isolation switch or a transmit-receive grating in conjunction with a delay line. Another approach divides a frequency spectrum into a set of orthogonal sub-bands by using coding techniques such as orthogonal frequency division multiplexing and bit loading.
In addition to these approaches, there are a variety of conventional techniques for isolating two or more antennas from one another by decoupling beam patterns of the antennas. Such techniques include modifying a directivity of the beam patterns (by antenna design and/or antenna placement), increasing a free space path loss (by physically separating the antennas), EMI shielding, one or more ground planes and, if possible, polarization isolation. While these techniques can improve antenna isolation, there are limits to the overall efficacy. In addition, there are inevitable antenna and communications system design tradeoffs. For example, to be effective, ground planes tend to have a large spatial extent. Such large ground planes add expense, are unwieldy (especially in compact and/or portable communications systems) and may restrict degrees of freedom in antenna design.
There is a need, therefore, for low cost and compact structures to increase the isolation of antennas in communications systems.